1. Field of the Invention
The present invention relates to an electrophotographic photoconductor which has high durability, high image quality and improved image stability over a long period of time, to an electrophotographic apparatus using the same, and to a process cartridge using the same.
2. Description of the Related Art
In an electrophotographic apparatus using the electrophotography method, such as a copying machine, facsimile, laser printer or direct digital machine, a toner image is formed on an electrophotographic photoconductor (also referred as photoconductor, hereinafter), mainly by charging, irradiating light imagewisely so as to form a latent electrostatic image and developing the latent electrostatic image. Thereafter, the toner image is transferred to a transfer medium such as a piece of paper, and then fixed on the paper. Further, the residual toner and the like on the surface of the photoconductor are removed so as to clean. By repeating this process, the image is formed on the paper.
As a photoconductor for the electrophotographic apparatus, such photoconductors are conventionally known in the art as the one containing a photoconductive layer having selenium or a selenium alloy on an electroconductive support, the one dispersing an inorganic photoconductive material such as zinc oxide or cadmium sulfide, and the one utilizing an amorphous silicone material. In recent years, due to advantages of cost, productivity, degree of freedom of photoconductor design and absence of pollution, organic photoconductors have come into wide use.
Organic photoconductors known in the art include a type of utilizing photoconductive resins such as polyvinyl carbazole (PVK), a charge transfer complexes type such as PVK-TNF (2,4,7-trinitrofluororene), a pigment dispersion type such as phthalocyanine-binder resins, and a separate function type combining a charge-generating material with a charge transport material. Of these, photoconductors of the separate function type are most common.
In a mechanism of latent electrostatic image formation in this separate function type of photoconductor, when the photoconductor is charged and exposed to light, the light passes through the transparent charge transport layer and is absorbed by the charge-generating material in the charge-generating layer. The charge-generating material, which absorbed the light, generates a charge carrier. This charge carrier is implanted into the charge transport layer, moves toward a surface of the photoconductor within the charge transport layer, under an electric field, and forms an electrostatic latent image by neutralizing the charge on the surface of the photoconductor.
In order to allow the charge to move, a charge transport material is added to the charge transport layer formed on the surface of the organic photoconductor. In general, the charge transport material is a low molecular weight compound, but as it does not have film-forming properties itself, it is usually formed by dispersing and mixing in an inert high polymer. However, the charge transport layer, which is formed of the charge transport material and inert polymer, is generally insufficient in hardness, and when used repetitively, it easily becomes worn due to the effect of charging, developing and cleaning. This low wear resistance is recognized to be a problem. The wear resistance is improved by increasing the proportion of inert polymer to the charge transport material, but as this causes the deterioration of sensitivity and the rise of residual potential, there is therefore a limit to increase wear resistance.
In recent years, according to increasing demands for a more compact electrophotographic apparatus, there is a trend towards smaller photoconductor diameters. Moreover, there have also been demands for higher speed, full color operation and maintenance free operation of electrophotographic apparatus. Thus, higher wear resistance of photoconductor becomes necessary. The wear resistance of an organic photoconductor may be improved by improving a binder resin (Japanese Patent Application Laid-Open (JP-A) No. 05-216250), or by including a polymer charge transport material (JP-A No. 51-73888, JP-A No. 54-8527, JP-A No. 54-11737, JP-A No. 56-150749, JP-A No. 57-78402, JP-A No. 63-285552, JP-A No. 64-1728, JP-A No. 64-13061, JP-A No. 64-19049, JP-A No. 03-50555, JP-A No. 04-175337, JP-A No. 04-225014, JP-A No. 04-230767, JP-A No. 05-232727, JP-A No. 05-310904). However, in the above-mentioned official publications, a low molecular weight charge transport material must be added even if the binder resin of the charge transport layer is improved, so the improvement of wear resistance is only slight. If a polymer charge transport material is used, on the other hand, wear resistance does improve due to the polymerization of the charge transport layer component, but the required properties is still not be able to be satisfied, and there is also a problem of cost and productivity, so it has not yet been realized.
An organic photoconductor having a protective layer provided on the charge transport layer, to which a wear-resistant function was added, has now come into wide use. For example, a method of adding a very hard metal oxide to the protective layer has been disclosed (JP-A No. 04-281461). In this method, residual electric potential and image blurring increase depending on the type of metal oxide added, but it is known to be effective for increasing wear resistance. Also, a method of crosslinking the protective layer has also been disclosed (JP-A No. 56-48637). This method has the side effect of a residual potential increase since the polymerization initiator and unreacted groups remain, but it has been found that this method offers the possibility of high wear resistance depending on crosslinking conditions and methods thereof.
Thus, there have been many attempts to improve wear resistance and achieve high durability of the organic photoconductor, which have now resulted in a remarkable increase of wear resistance. However, the increase of wear resistance has been associated with problems of considerable image defects, such as image blurring. This image blurring is due to a drop in the surface resistance of the photoconductor, which causes charge to move horizontally leading to blurring of the latent electrostatic image. It is thought that the decrease of surface resistance is caused by ions (hereafter referred to as discharge products) due to the interaction of ozone and NOx gas produced when the photoconductor is charged, with moisture in the air, and they are attracted to and deposit on the photoconductor. Also, external additives in the toner are attracted to and deposit on the photoconductor surface, and paper particles attracted to and deposit on it during transfer. These are also known to be the factors of leading the image defects including image blurring. The image defects were not a major problem on a conventional photoconductor because it had low wear resistance, so even if foreign matter, such as discharge products, toner external additives or paper particles, were not attracted to the photoconductor surface, they were removed due to wear. However, now that the wear resistance of the photoconductor has been improved, removal of this foreign matter is difficult, and the image defects tend to appear in an early stage.
Although it is now possible to improve wear resistance of the photoconductor, if image defects such as image blurring, or deterioration of image quality is more likely to occur, it does not mean that high durability has been achieved. To reduce these image defects, a method has been proposed to reduce the surface energy and frictional coefficient on the photoconductor surface, and another method has been proposed where a dehumidifier is provided which heats the photoconductor.
A method is known for reducing the surface energy and frictional coefficient on the photoconductor surface by adding various lubricants to the surface layer of the photoconductor. Methods of adding lubricants such as fluorinated silicone oil to the surface layer are disclosed in JP-A No. 07-295248, JP-A No. 07-301936 and JP-A No. 08-082940. Although this method is recognized to be effective for cleaning or removing foreign matters by reducing the surface energy of the photoconductor, these fluorinated silicone oils migrate near the surface during the formation of the protective layer, so the effect will be lost at an early stage due to small amounts of wear on the surface layer after repeated use. Therefore, this did not have much effect on increase of durability.
Moreover, various methods have been attempted to add particulate lubricants to the outermost layer of the photoconductor, for example, addition of silicone resin particles or fluorine-containing resin particles (JP-A No. 63-65449), or of melamine resin particles (JP-A No. 60-177349). JP-A No. 02-143257 discloses a method of adding polyethylene fine particles to the surface layer, JP-A No. 02-144550 discloses a method of adding fluorine-containing resin fine particles to the surface layer, JP-A No. 07-128872 and JP-A No. 10-254160 discloses a method of adding silicone particles to the surface layer, furthermore, JP-A No. 2000-010322 and the U.S. Pat. No. 5,998,072 discloses a method of adding crosslinked organic particles to the surface layer. Further, JP-A No. 08-190213 discloses a method of adding methyl siloxane resin particles to the surface layer. The dispersion of these particulate lubricants in the surface layer of the photoconductor is effective to improve durability, and it can be said to be more effective than addition of silicone oil for increasing durability. However, since the effect is maintained when the photoconductor surface wears out to some extent, there was a problem in that it was not effective in a photoconductor having increased wear resistance. If these particulate lubricants were covered with the binder resin, the effect was not demonstrated at all, so the photoconductor surface had to be worn down beforehand, and this method was therefore difficult to apply to a photoconductor with improved wear resistance.
The addition of these particulate lubricants caused a decrease of optical transmittance of the protective layer, or increase of residual potential leading to image deterioration, whereas if the addition amount was limited, sufficient durability was naturally not obtained. The addition of these particulate lubricants also tended to reduce the hardness of the photoconductor, which was detrimental to increasing durability. In addition, as these particulate lubricants had mold-release properties, their compatibility with the binder resin or organic solvent was very poor, so aggregation increased and there was a strong tendency to poor dispersibility. Low dispersibility of particulate lubricants leads to loss of optical transmittance, poor layer quality and loss of layer surface flatness. This not only promotes image deterioration, but also loss of homogeneity of the particulate lubricants in the layer. Thus, the reduction of surface energy could not be maintained and stability fell sharply, which adversely impacted continuity of effect.
Thus, although the wear resistance of photoconductors has been improved, image defects such as image blurring are a major problem, and this is currently the largest obstacle to increasing the durability of the photoconductor. If it is attempted to remove the foreign matter which causes image blurring, the wear resistance of the photoconductor falls, and the soiling of the photoconductor surface is worse, the higher the wear resistance is. Thus, it was extremely difficult to increase the wear resistance and prevent image defects at the same time.
To resolve this problem, a method of dehumidifying the photoconductor by heating is sometimes used. As image blurring is considered to be due to discharge products absorbing the moisture in the air, image blurring can be suppressed by dehumidifying the photoconductor surface. However, this method had many problems. For example, the photoconductor had to be heated continuously, power consumption increased by a large amount, and it required a long time to start the apparatus. The electrophotographic apparatus had to include a dehumidifier for heating the photoconductor, and as it was difficult to apply the dehumidifier to the small diameter photoconductors which have come into use in recent years, the electrophotographic apparatus was necessarily bulky.
Another solution to this problem is to apply particulate lubricants to the photoconductor surface. The continuous application of lubricant to the photoconductor surface is effective in maintaining continuity of the surface energy lowering effect regardless of the amount of wear on the photoconductor, and thus adhesion of foreign matter can be stably prevented over a long period of time. However, due to the inclusion of a step for applying lubricant to the photoconductor surface in the electrophotography process, the electrophotographic apparatus again became bulkier, and it was hard to apply to small diameter photoconductors which have come into use in recent years. In addition, lubricant was necessary to be supplied over time, and it was difficult to set the lubricant application amount. If the application amount was excessive, it occasionally caused image blurring and defective cleaning, or image defects such as thinning of characters, so many problems still remained.
In order to manufacture an electrophotographic apparatus which does not require replacement of the photoconductor or a dehumidifier, and which achieves high durability, high image quality, compactness and energy saving, the wear resistance of the photoconductor must be increased while at the same time, the foreign matter adhesion responsible for image blurring and image defects must be reduced. In particular, in electrophotographic apparatuses using a small diameter photoconductor which have become popular in recent years, since it is difficult to include a dehumidifier or lubricant supply, it was desired to incorporate these functions in the photoconductor itself. However, if the lubricants of the prior art were added to the photoconductor surface to implement these functions, they caused a decrease in the optical transmittance of the layer, less resistance to scratches, lower strength, higher residual potential, lower layer quality and reduced surface flatness. Hence, continuity of the surface energy reduction effect was inadequate, and this aggravated image deterioration. Lubricants added to reduce surface energy have the effect of increasing mold release properties, so their compatibility with the binder resin or organic solvent forming the surface layer was poor, and clumps tended to form. This reduced layer quality and surface flatness, led to less continuity of the surface energy reduction effect, and was considered to be a major factor interfering with the improvement of durability.